Water / Cement ratio – Abram’s Law – Gel/space ratio – Gain of strength of concrete – Maturity concept

ELASTICITY, CREEP & SHRINKAGE – Modulus of elasticity – Dynamic modulus of elasticity –
Posisson’s ratio – Creep of concrete – Factors influencing creep – Relation between creep & time –
Nature of creep – Effects of creep – Shrinkage – types of shrinkage.

1. PRINCETON INSTITUTE OF ENGINEERING AND TECHNOLOGY FOR WOMEN-HYDERABAD. DEPARTMENT OF CIVIL ENGINEERING C.T-UNIT-4 Hardened Concrete: Presented by: MOOD NARESH ASST.PROF PETW

2. What is Creep of Concrete? Creep of concrete is the permanent deformation reported in a part of a structure due to the sustained load for a long time. Creep indicates that strain in the structure continuously increases, but stress remains constant. Creep in a structure will depend on the structural material, loading quantity, loading duration, etc. Creep in concrete structures will be lesser than in steel structures. Creep deformation generally occurs in 3 stages. These stages are called primary creep, secondary creep and tertiary creep . In the primary stage, concrete creep starts rapidly, and the rate decreases with time . In the secondary stage, creep generally occurs at a constant rate . And in the tertiary stage, creep starts at an increasing rate until the structural member breaks down.

3. Elastic modulus Elastic modulus is a fundamental parameter in designing concrete structures. In recent years building specifications have even required a specific elastic modulus of concrete to be met, mostly to limit excessive deformation and sway in tall buildings. For Burj Khalifa (currently the tallest building in the world), the designer specified minimum 43800 MPa for 80 MPa concrete mixes for the vertical elements . In the simplest terms, modulus of elasticity (MOE) measures the stiffness of the material and is a good overall indicator of its strength. It is the ratio of stress to strain. Stress is the deforming force acting per unit area (F/A), and strain is the deformation (change in shape) caused by stress (∆L/L).

4. Based on the types of stress (tension, compression or shear) and strain, including the direction, different types of elastic modulus can be identified as detailed below. 1.      Young’s Modulus (E) - ratio of linear stress to linear strain, 2.      Shear Modulus (G or µ) – ratio of shear stress to shear strain and, 3.      Bulk Modulus (K) – ratio of volume stress to volume strain.

5. Units of elastic modulus are the followings: In SI unit  MPa  or N/mm 2  or KN per square meter. In FPS unit psi or  ksi   or psf or ksf .

6. Water is measured by its volume and it is specified as no of litres per bag of cement. The further increase or decrease in water to be mixed, depends on the quantity of water present in sand and coarse aggregates, due to Hygroscopic action or any other reason. The water present in aggregates should be subtracted from the total quantity of water required for the mix to get the desired quantity of water needed. Similarly, if aggregates are dry and are likely to absorb water, extra water should be added to maintain the water content in the mix. Water-Cement ratio The ratio of the volume of water to the volume of cement used to form concrete is termed as water-cement ratio.

7. Optimum Water content This is the most suitable amount of water required for a given proportion of materials to gain its maximum strength. If the water content is below the optimum water content, the process of setting of cement will not be completed properly. Thus, there will be a decrease in the strength of concrete. Similarly, if the water content is above the optimum water content, the workability of concrete will increase but the strength will decrease. From studies and experiments, it is found that if the water is increased by 10% above the optimum water content the strength of concrete reduces by 15%, and the increase in 50% water content reduces the strength of concrete by 50%.

8. Abram's Law Abram’s law states that the strength of concrete mix entirely depends on the water-cement ratio. The increase in water-cement ratio decreases the strength of the mix i.e.  'the strength of concrete is inversely proportional to the water-cement ratio'. Abram’s Law is a special case of general rule formulated by  FERET  with his experience and experiments. Where,  S = Compressive Strength of concrete A & B are empirical constants (w/c) is water-cement ratio [Note] Duff Abrams suggested empirically the values for  A  as 14000 and  B  as 7 in F.P.S. system and  A  as 984 and  B  as 7 in M.K.S. units .

9. Nondestructive Testing of Concrete (NDT):  Nondestructive testing of concrete (NDT) is a testing method used by industry to determine the concrete compressive strength, defects and discontinuities without causing damage to the original part of an existing structure . Nondestructive Testing of Concrete Objective : The test aims to assess the strength and other properties and to locate and obtain corresponding results showing permeable areas, cracks or laminations and areas of lower integrity than the rest. 

10. List of Nondestructive Testing Methods: Various methods of nondestructive testing of concrete have been developed and are under development for examining different properties of concrete in addition to the necessary visual inspection.  It is essential to highlight that it is unnecessary to carry out all the nondestructive testing of concrete in each case except the most relevant ones. In this section, we will discuss the following nondestructive testing methods.

11. List of Nondestructive Testing Methods: Rebound Hammer Test Ultrasonic Pulse Velocity Test Covermeter Test Carbonation Test Concrete Core Cutting Test Half Cell Potentiometer Test In-situ permeability Test Pull out Test Windsor Probe Test

12. https://www.constructioncivil.com/nondestructive-testing-of-concrete/#gsc.tab=0

13. Concrete Rebound Hammer Test: Generally, Schmidt concrete test hammer is used for conducting the concrete rebound hammer test, which is nondestructive testing of concrete. The test is a beneficial and quick procedure where we can get a clearer idea about the compressive strength of existing concrete of a particular structure. The main principle of the test is based on the elastic mass rebound theory. The elastic mass rebound depends on the hardness of the existing concrete from where the mass impinges.  A spring-loaded mass has a fixed amount of energy imparted by extending a spring to a fixed position, and it is performed by pressing the plunger against the surface of the existing old concrete under test.  Upon release, the mass rebounds from the plunger, still in contact with the concrete surface. The distance travelled by the mass, expressed as a percentage of the primary extension of the spring, is known as the rebound number; it is shown by a rider moving along a graduated scale.  The test surface is prepared by rubbing the concrete surface with a carborundum stone.  Approximately ten hammer readings are noted around the test points. The test is conducted as per IS:13311(Part 2)-1992.

14. Concrete Rebound Hammer Test: Generally, Schmidt concrete test hammer is used for conducting the concrete rebound hammer test, which is nondestructive testing of concrete. The test is a beneficial and quick procedure where we can get a clearer idea about the compressive strength of existing concrete of a particular structure. The main principle of the test is based on the elastic mass rebound theory. The elastic mass rebound depends on the hardness of the existing concrete from where the mass impinges.  A spring-loaded mass has a fixed amount of energy imparted by extending a spring to a fixed position, and it is performed by pressing the plunger against the surface of the existing old concrete under test.  Upon release, the mass rebounds from the plunger, still in contact with the concrete surface. The distance travelled by the mass, expressed as a percentage of the primary extension of the spring, is known as the rebound number; it is shown by a rider moving along a graduated scale.  The test surface is prepared by rubbing the concrete surface with a carborundum stone.  Approximately ten hammer readings are noted around the test points. The test is conducted as per IS:13311(Part 2)-1992.